This invention relates to a prosthesis for use in the surgical correction of certain mitral or tricuspid valve disorders. There are two atrio-ventricular valves in the heart. That on the left side of the heart known as the mitral valve, and that on the right side known as the tricuspid valve. Both valves are subject to damage that requires that the valves be repaired or replaced. Clinical experience has shown that repair of the valve, where this is technically possible, produces better long term results than does valve replacement. The mitral and tricuspid valve differ significantly in anatomy. Whereas the annulus of mitral valve is somewhat "D" shaped, the annulus of the tricuspid valve is more nearly circular.
The effects of valvular dysfunction vary. Mitral regurgitation has more severe physiological consequences to the patient than does tricuspid valve regurgitation, a small amount of which is tolerated quite well. In patients with valvular insufficiency it is increasingly common surgical practice to retain the natural valves, and to attempt to correct the defects. Many of the defects are associated with dilation of the valve annulus. This dilatation not only prevents competence of the valve but also results in distortion of the normal shape of the valve orifice. Remodelling of the annulus is therefore central to most reconstructive procedures on the mitral valve.
Many procedures have been described to correct pathology of the valve leaflets and their associated chordae tendinae and papillary muscles. In mitral repairs it is essential to preserve the normal distance between the two fibrous trigones. The trigones almost straddle the anterior leaflet portion of the annulus. Between the left and right fibrous trigones the mitral annulus is absent (as described by Tsakiris AG. "The physiology of the mitral valve annulus" in The mitral valve-a pluridisciplinary approach. ed Kalmanson D. Publishing Sciences Group, Acton, Mass. 1976, pg 21-26). This portion of the mitral valve apparatus is formed by the change of the anterior portion of the base of the aorta into the (so called) sub-aortic curtain, and hence into the anterior leaflet of the mitral valve. A significant surgical diminution of the inter-trigonal distance could cause left ventricular outflow obstruction. Thus it is highly desirable to maintain the natural inter-trigonal distance during and following mitral valve repair surgery. Consequently, when a mitral valve is repaired (be it the posterior or anterior leaflet) the result is generally a reduction in the size of the posterior segment of the mitral valve annulus.
As a part of the mitral valve repair it is either necessary to diminish (i.e. constrict) the involved segment of the annulus so that the leaflets may coapt correctly on closing, or to stabilize the annulus to prevent post-operative dilatation from occurring. Either is frequently achieved by the implantation of a prosthetic ring in the supra annular position. The purpose of the ring is to restrict and/or support the annulus to correct and/or prevent valvular insufficiency. However, it is important not to over restrict the annulus or an unacceptable valvular stenosis would result. As described above, in mitral valve repair, constriction of the mitral annulus should take place only in the area of the posterior section of the valve annulus. Shortening of the posterior portion of the mitral valve annulus may be accomplished in several ways. Firstly, by implanting a substantially inexpansible ring (smaller in size than the annulus). With this type of device, the surgeon must accurately choose the size of ring that will just prevent insufficiency, yet will not cause significant valvular stenosis. Secondly, by a using a contractable ring that may be plicated during implantation. This type has the disadvantage that the surgeon must then accurately judge not only the ring size to use, but also how to space the implanting sutures in the ring and the annulus so that when implanted, insufficiency is minimized, yet there will be no significant valvular stenosis. Thirdly, and preferably, by a substantially inexpansible ring that may be contracted only in appropriate segments (and not in the anterior portion). The natural inter-trigonal distance should be maintained, and the anterior leaflet should not be diminished in circumference.
In tricuspid valve repair, constriction of the annulus usually takes place in the posterior leaflet segment and in a small portion of the adjacent anterior leaflet. The septal leaflet segment is not usually required to be shortened.
Various prostheses have been described for use in conjunction with mitral or tricuspid valve repair. Each has disadvantages. The ring developed by Dr. Alain Carpentier (U.S. Pat. No. 3,656,185) is rigid and flat. Although widely used, criticism of its inflexibility preventing the normal alteration in size and shape of the mitral annulus with the cardiac cycle has been widespread. The complication of left ventricular outflow tract obstruction has been described in association with this device. This complication can take the form of a decrease in the dimensions of the left ventricular outflow tract, or systolic anterior motion of the anterior leaflet of the valve. Both complications were reported by Geller M, Kronzon I, Slater J et al. "Long-term follow-up after mitral valve reconstruction: incidence of postoperative left ventricular outflow obstruction". Circulation 1986;74(suppl I) I-99-103. They implanted Carpentier rings in sixty-five patients. All sixty surviving patients were restudied 1-55 months postoperatively. All showed a significant decrease in the dimensions of the left ventricular outflow tract, and 6 patients (10%) also had systolic anterior motion (SAM). Another complication of the Carpentier ring has been inflow obstruction. This complication associated with its use in tricuspid valves was reported by Carpentier et al. in nine of seventeen patients (Carpentier A, Deloche A, Hanania G, et al. "Surgical management of acquired tricuspid valve disease". J Thorac Cardiovasc Surg 1974;67:53-65). In addition, the Carpentier ring has the disadvantage of not being of adjustable size. Thus the surgeon has to accurately judge the correct size of ring needed to reduce the annulus size and produce a competent valve.
An open ring valve prosthesis was described in U.S. Pat. No. 4,164,046 comprising a uniquely shaped open ring valve prosthesis having a special velour exterior for effecting mitral and tricuspid annuloplasty. This ring was not adjustable in size during or following implantation. The fully flexible annuloplasty ring described by Carlos D. Duran and Jose Luis M. Ubago, "Clinical and Hemodynamic Performance of a Totally Flexible Prosthetic Ring for Atrioventricular Valve Reconstruction" Annals of Thoracic Surgery, (No. 5), 458-463, (November 1976) could only be shortened in the posterior segment by the placement of plicating sutures. The judgement of the position, size and spacing of these sutures requires skill and experience. However, inappropriate suture placement in the anterior segment could cause undesirable intra-trigonal shortening. Adjustable annuloplasty rings were described by Dr. William Angell (U.S. Pat. No. 4,042,979) and Dr. Miguel Puig-Massana (U.S. Pat. No. 4,290,151). Both incorporate draw-strings capable of reducing the size of the posterior portion of the ring. The former contains a rigid or flexible member in the anterior leaflet portion of the ring. The latter ring is also adjustable but fully flexible. With this device the use of a continuous implantation suture was recommended rather than the more generally used interrupted sutures. With the Puig-Massana ring the use of interrupted sutures would be likely to interfere with the internal drawstrings. However, should a continuous suture be used for implantation, and the ring then contracted by the internal drawstrings, loosening of the continuous suture would be caused by the reduction in the circumference of the annulus. A further disadvantage of Puig-Massana's ring is that following the tightening of the drawstrings, a bulky knot is formed on the atrial surface of the ring. Hence, the knot lies in the direct blood flow path into the inflow of the valve. Should a thrombus form on the knot it could later embolize. In addition, should the surplus drawstrings be cut too close to the knot, there is the danger of the knot becoming undone. Conversely, should significant surplus drawstrings tails remain, abrasion of the valve leaflets could occur. The adjustable ring by Dr. Ali Ahmadi has the disadvantage of being circular, which is not an appropriate shape, particularly for the mitral annulus.
The rigid rings described above were probably conceived on the assumption that the mitral annulus is "D" shaped and lies in a single flat plane. That this was a misconception was shown by Levine, R. A., Triulzi, M. O., Harrigan P., and Weyman, A. E. "The relationship of mitral annular shape to the diagnosis of mitral valve prolapse", Circulation 75, No. 4, 756-767, 1987. This work shows that the mitral valve annulus is a complex and mobile structure and demonstrated that the mitral valve takes the form of a central, elliptical portion of a hyperbolic paraboloid or saddle shaped surface. It is clear that imposing a flat ring or even a segment of a flat ring would distort the annulus and could cause left ventricular outflow tract obstruction. The device which is the subject of this invention does not have these disadvantages.